Bethanechol enantiomers?

• A. S enantiomer is more active than R
• B. R enantiomer is more active than S
• C. They have equal activity
• D. Both inactive

Answer: (A)

The activity difference comes from how a chiral drug fits a chiral receptor. Bethanechol is a muscarinic receptor agonist, and its two enantiomers don’t bind with the same fit or strength to the receptor’s binding pocket. The S configuration places the quaternary ammonium–bearing part of the molecule in a orientation that interacts optimally with the receptor, producing greater binding affinity and activation. The R enantiomer is misaligned, so its affinity and potency are lower. That’s why the S enantiomer is more active than the R enantiomer. The idea that they have equal activity or that both are inactive doesn’t match the typical enantiomer-specific potency seen with receptor-mediated actions like bethanechol.